The present invention relates to powder metallurgy and more particularly to a process of producing nonevaporable getter materials and to getters manufactured therefrom, featuring enhanced mechanical and sorption properties.
Nonevaporable getters are well-known in the field of vacuum technology, and have been successfully used therein for more than thirty years for the provision and maintenance of a high vacuum level in different devices where vacuum is required: kinescopes, thermal insulation vessels and cathode-ray tubes, in elementary particle sources and accelerators (the thermonuclear fusion reactor of the TOKAMAK T-15 type) or the LEP (Large Electron-Positron) accelerator at CERN in Geneva, where the use of NGs makes it possible to reach a residual pressure below 10xe2x88x9210 Pa. Another broad field of NG application is the purification of inert gases. The best-known nonevaporable getters are alloys: Zr-Al, containing 84 weight % Zr, described in U.S. Pat. No. 3,203,901; a ternary alloy, having the composition 70 weight % Zr, 24.6 weight % V, and 5.4 weight % Fe, described in U.S. Pat. No. 4,312,669; and an intermetallic compound ZrMnFe, described in U.S. Pat. No. 5,180,568. Getter elements are manufactured mainly from powders whose particle size varies from several microns to several hundreds of microns. Since loose powders in most cases can be used as getter elements, such powders are pressed into articles of different shapes (tablets, washers, disks, etc.) or rolled into strips. Porous getters with high sorption properties are manufactured as disclosed in U.S. Pat. No. 4,428,852; U.K. Patent No. 2,077,487; and German Patent No. 2,204,714.
In the information sources cited above, the getter material is produced by melting and subsequent crushing of the ingot down to powder; getters produced from these powder materials possess low mechanical properties.
Known in the art are getters made from powder alloys, described in R.F. Patent No. 1,649,827 -a Zrxe2x80x94Vxe2x80x94Ca composition; in R.F. Patent No. 2,034,084 -a Tixe2x80x94Crxe2x80x94Ca composition; and in R.F. Patent No. 1,750,256, which is the closest in terms of the technical solution, the latter comprising preparation of powders for getter materials having the composition Tixe2x80x94Vxe2x80x94Ca by reducing a mixture of Ti and V oxides with calcium hydride in accordance with the main reaction:
MeO+CaH2xe2x86x92Me+CaO+H2↑+Q kcalxe2x80x83xe2x80x83(1).
The reaction product is a mixture of powders of metals and CaO, sintered into a briquette (xe2x80x9csinterxe2x80x9d). This xe2x80x9csinterxe2x80x9d is then crushed and treated with hydrochloric acid to separate the metal powder from CaO; after that the powder is shaped. The reducing temperature is 175xc2x0 C. held for 6 hours, and the resulting finished product is believed to be a powder alloy. However, an in-depth study showed that the above Tixe2x80x94Vxe2x80x94Ca composition is chemically heterogeneous and comprises predominantly a mixture of almost pure metallic particles which have not reacted with each other, and owing to such a high and non-regulated degree of chemical heterogeneity this getter material, though displaying a sufficiently high level of chemical properties with respect to all the above-mentioned materials, has insufficiently high gas sorption properties. In the prior art method, the reduction conditions, as well as non regulated conditions of shaping and sintering the metal powder, do not allow producing articles with equally high mechanical and sorption properties. In the prior art no information could be found on the interrelation of the mechanical and sorption properties of the getter with its chemical heterogeneity.
For the getter to meet all the requirements imposed on it, it must have very good mechanical properties along with high sorption characteristics with respect to such gases as H2, O2, N2, CO, and the like. Low plasticity and strength do not provide sufficient resistance to mechanical loads and stresses caused by the processes of heat-cycling in the range from 300-700xc2x0 C. to the ambient temperature. All this leads to disintegration of getters into separate fragments or to their crumbling, which cannot be tolerated in vacuum systems, e.g., in vacuum tubes, in elementary particle sources and accelerators, whereas low sorption properties cannot provide long-time maintenance of a residual pressure on the order of less than 10xe2x88x9210 Pa.
Therefore, the provision of getters noted for a combination of improved mechanical and sorption properties is an urgent problem. An extension of the range of materials used in the production of getters is a no less urgent problem.
In the proposed group of inventions the first embodiment solves the problem of providing getter material; the second embodiment relates to the getter produced, which combines enhanced mechanical and sorption properties. Investigations showed that a combination of enhanced mechanical and sorption properties is provided due to the definite degree of chemical heterogeneity of the getter material, the zones of relatively pure plastic metals which enter into the composition of the material and have poorly reacted with each other being responsible for the mechanical properties, and the zones of their interaction being responsible for the sorption activity level.